﻿part 2] 



THE I SLAY ANTICLINE. 



145 



which, according to Dr. Peach and Mr. Wilkinson, exists between 

 the Portaskaig Conglomerate and the Islay Limestone. Unfor- 

 tunately, the figure is wrongly drawn, for it shows the basal layer 

 of the conglomeratic series obliquely truncating the bedding of the 

 limestone. This is not the case : for, although a plane of discordance 

 separates the two groups, the conglomerate and limestone have 

 identically the same dip. The plane of discordance may, perhaps, 

 have been determined by erosion, as advocated in the memoir ; but, 

 if so, the section gives no clue as to which of the two groups is 

 unconformable to its neighbour. 



The Grarvellachs, or Isles of the Sea, — The oldest rock 

 exposed in the Isles of the Sea is a cream-coloured, Avhite, or 

 pinkish dolomite — or dolomitic limestone — accompanied in places 

 by light- grey slaty beds. Dr. Peach has estimated the thickness 

 of the dolomite as not less that 40 feet. The succeeding rocks are 

 almost all conglomeratic. 



On the north-western shore of the main southern island, Eileach 

 an Naoimh, Dr. Peach discovered a beautifully-exposed anticline 

 of the dolomite, surmounted hj bedded conglomerate made up 

 exclusively of dolomite fragments, many of them of great size 

 (fig. 4, p. 146). Farther out from the anticlinal axis the dolomite 

 fragments are smaller and enclosed in a dark matrix ; while well- 

 rounded boulders of rocks foreign to the islands — nordmarkite, 

 felsite, gneiss, schist, jasper, etc. — make their appearance, some- 

 times in great abundance. Then follow intercalations of sandy 

 shales, with bands of sandy dolomite and flaggy fine-grained 

 quartzite. These are succeeded by brown-weathering conglo- 

 merates charged with dolomite- and nordmarkite-boulders. Pure 

 quartzite is interbedded with the conglomerate, and becomes 

 increasingly prominent at a distance from the anticlinal axis. It 

 contains boulders similar to those that have been enumerated 

 above, but more sporadically distributed. 



The main Grarvellach dolomite, which yields so many fragments 

 to the Portaskaig Conglomerate in its vicinity, recalls in appear- 

 ance the dolomite intercalations of the conglomeratic series in 

 Islay, only it is thicker and purer. It is impossible, without 

 boring, to decide whether the Garvellach dolomite is similarly 

 interstratified, or whether it completely underlies the conglomeratic 

 sequence. The interstratified dolomites in Islay, it will be remem- 

 bered, have suffered from contemporaneous erosion, and therefore 

 the erosion effects in connexion with the Grarvellach bed are in 

 keeping with either alternative. 



(3e) Islay Quartzite. 



North Islay. — The Islay Quartzite is not wholly represented 

 in North Islay, as the top is wanting. In spite of this, North 

 Islay is the most interesting portion of the archipelago from our 

 point of view, for it enables us to realize the anticlinal nature of 



